The concept of “digital cinema” includes the production, delivery and presentation of aural/visual material in auditoriums or theatres using digital technology. Digital cinema programs typically are distributed in a compressed and encrypted form on physical media such as DVD-ROM, tape or computer hard drives and can in principle be distributed by electronic transmission using satellite or other broadband communication paths.
Digital cinema playback systems control the processes required to make a digital cinematic presentation. These processes include receiving and storing the digital cinema program, decompressing and deciphering it into digital video and audio bit streams that can be processed by digital content decoders, decoding the content of the bit streams to obtain signals that may be used drive video displays and audio amplifiers, and controlling other facilities such as special effects, curtains and theatre lighting that are found in a theatre auditorium.
Typical digital cinema playback systems include several pieces of equipment, devices, that communicate with one another through an electrical network that is similar to many networks that are used to interconnect computers. These networks often conform to a standard that is commonly known as Ethernet, which is described in the IEEE 802.3 standard, using a communication protocol known as the Transmission Control Protocol/Internet Protocol (TCP/IP). This choice of network and protocol can simplify the task of implementing a digital cinema playback system because the electrical and logical interfaces and procedures needed to use them are readily available and have relatively low cost.
In a digital cinema playback system, the devices on the network must be synchronized to each other, and to a master clock in a fashion analogous to that in a television broadcast facility. Typically, in a broadcast facility a master video clock signal source provides a reference for the entire facility. This signal, which is usually a video frame reference clock, is routed to all of the servers, tape machines, etc. via a dedicated hardware interface and provides an accurate timing signal. In addition, SMPTE timecode signals are used to synchronize multiple servers/tape machines to the same video frame. The SMPTE timecode is described in the SMPTE 12M design (Society of Motion Picture and Television Engineers, Recommended Practice 12M: 1999).
In a digital cinema playback system, the timing requirements are not as strict, but at least frame accurate timing is required among the networked devices, including the server, decoder, and automation devices.
A technology to synchronize real time clocks in computers on a network has been in use for a number of years. This is called Network Time Protocol, or NTP. Detailed specifications of version 3 of NTP are contained in “Network Time Protocol (Version 3) Specification, Implementation and Analysis,” Network Working Group, Request for Comments: 1305, March, 1992. Although NTP has been used to synchronize the real time clocks in a broadcast station, NTP clocks are not used to synchronize frame dependent events, nor to ensure video frame synchronization across devices such as tape machines, servers, etc. Such frame synchronization has been provided in broadcast environments by dedicated hardware interfaces and/or additional device-interconnecting cables.
It would be desirable to provide the required video frame synchronization in a digital cinema playback system within an Ethernet interface and without the need to have such dedicated hardware interfaces and/or additional cables.